Image matching is a technique that matches an image against one or more reference images to determine whether or not the image matches any of the reference images. Image matching technique of this type is used in matching a facial image or a fingerprint image captured by an image pickup device against registered images stored previously in a database, for example. In this example, the facial image and fingerprint image are images to be matched and the registered images are reference images. In conventional biometric image matching techniques, global structural features unique to a living individual (for example, the eyes, the eyebrows, and the mouth) are matched. Since global structural features are fixed in number and are in almost fixed positions, matching based on global structural features can be performed with relative ease.
However, it is difficult to perforin matching of images of living individuals that have extremely similar global structural features, such as a twin, with a high degree of accuracy with the matching technique. Therefore, matching techniques based on acquired local structural features (for example, skin traits such as moles, freckles and wrinkles, and fingerprints), in addition to global structural features have been proposed.
Related-art documents concerning the image matching technique include JP2006-107288A (hereinafter referred to as Patent document 1), JP06-28461A (hereinafter referred to as Patent document 2) and JP2005-521975 (hereinafter referred to as Patent document 3).
Patent document 1 discloses a personal verification technique in which skin texture such as moles, flecks and freckles are detected and the detected pattern is matched against feature patterns registered in a database. Patent document 2 discloses a fingerprint matching technique in which a window image is extracted on the basis of a global structural feature in a fingerprint image and matching is performed on feature points (minutiae) such as branches and ends of a fingerprint that appear in the window image. Patent document 3 discloses a personal verification technique in which a reference image is searched, a group of pixels that is the best match for a group of pixels in an object image (acquired image) is selected, and the probability that the relative locations of the selected group of pixels and the group of pixels in the object image randomly occur is determined.